^{3}.

Nuclear stopping in central Au+Au collisions at relativistic heavy-ion collider (RHIC) energies is studied in the framework of a cascade mode and the modified ultrarelativistic quantum molecular dynamics (UrQMD) transport model. In the modified mode, the mean field potentials of both formed and “preformed” hadrons (from string fragmentation) are considered. It is found that the nuclear stopping is increasingly influenced by the mean-field potentials in the projectile and target regions with the increase of the reaction energy. In the central region, the calculations of the cascade model considering the modifying factor can describe the experimental data of the PHOBOS collaboration.

The study of strongly interacting matter at extreme temperatures and densities is provided a chance by heavy-ion collisions at ultrarelativistic energies [

The main purpose of this work is to extract the information on nuclear stopping by comparison of the pseudorapidity distributions of charged particles from a transport-model simulation with data. This goal can be achieved by studying the pseudorapidity distribution of charged particles from central Au+Au collisions at relativistic heavy ion collider (RHIC) energies, within a transport model, the ultrarelativistic quantum molecular dynamics (UrQMD) model. And the modifying factor is considered in this cascade mode. This method is advantageous to directly compare existing data, and it can describe the experimental data very well.

The UrQMD model is a microscopic many-body transport approach and can be applied to study proton-proton (pp), proton-nucleus (pA), and nucleus-nucleus (AA) interactions over an energy range from the heavy-ion synchrotron (SIS) to RHIC. This transport model is based on the covariant propagation of color strings, constituent quarks, and diquarks (as string ends) accompanied by mesonic and baryonic degree of freedom [

The UrQMD model is based on parallel principles as the quantum molecular dynamics (QMD) model; hadrons are represented by Gaussian wave packets in phase space and the phase space of hadron

Here,

In the modified version of UrQMD (based on the version 3.3), the following two terms are further added: (1) the density-dependent symmetry potential term

It is known that the calculation results of the UrQMD model about the (pseudo)rapidity distribution of charged particles are somewhat larger than that of the experimental data [

At RHIC energies, as the pseudorapidity distribution of charged particles in the transverse direction has not been provided by experimental physicists, we study the nuclear stopping with the longitudinal pseudorapidity distribution. Figures

Charged particle pseudorapidity distributions for central Au+Au collisions at

Charged particle pseudorapidity distributions for central Au+Au collisions at

Charged particle pseudorapidity distributions for central Au+Au collisions at

In Figure

In Figure

In Figure

From these three figures, it is known that the value of modifying factor increases with the increase of reaction energy. At RHIC energies, with the growth of reaction energy, the regions are enlarging which is affected by the potentials of both formed and “preformed” hadrons. At

In summary, we have presented the pseudorapidity distribution of charged particles for central Au+Au reactions at RHIC energies. The UrQMD transport model (version 3.3) with the cascade mode and the modified mode (considering the potentials of both formed and “preformed” hadrons) has been used in all calculations. Based on this model we have investigated the effect of mean-field potentials on nuclear stopping. It is found that nuclear stopping is increasingly influenced by the mean-field potentials in the projectile and target regions with the increase of reaction energy. Since most of the incident energy is concentrated in the central region, the calculations of the cascade model considering the modifying factor can describe the experimental data in the projectile and target region. In conclusion, our combined calculations with the cascade mode and the modified mode in the different regions can give the agreement between calculation results and the experimental data. But the intrinsic reaction mechanism has yet to be studied in depth.

The authors declare that there is no conflict of interests regarding the publication of this paper.

The authors acknowledge support by the Cluster-Computing Center of School of Science (C3S2) of Huzhou Teachers College for Scientific Computing. This work is supported by the Natural Science Foundation of Guangxi Zhuangzu Autonomous Regions of China under Grant no. 2012GXNSFBA053011 and the Starting Foundation of Scientific Research of the Doctor of Guangxi Teachers Education University of China.